1. Field of the Invention
The present invention relates to a halogenated polyamide acid composition, a halogenated polyimide film obtained therefrom, an optical waveguide, and an optical waveguide device.
2. Description of the Related Art
Technologies with respect to an optical waveguide as its basic component have been marked in accordance with the practical use of an optical communication system. The optical waveguide typically has an embedded type structure in which a clad layer with a low refractive index surrounds a core layer with a high refractive index, or a ridge type structure in which a core layer with a high refractive index is formed on a lower clad layer with a low refractive index and an upper clad layer is made of an air layer, and light injected into the optical waveguide is transmitted in the core layer while being reflected on the interface between the core layer and the clad layer or on the interface between the core layer and the air layer.
As the material to form an optical waveguide, there have been known inorganic materials such as quartz glass and semiconductor. On the other hand, the research and development of producing an optical waveguide with various polymers have been carried out. Since coating and heat treatment can be carried out for polymers as organic materials under normal pressure at a film forming step contrary to the inorganic materials, there is an advantage that its apparatus and production steps can be simplified.
As the material of a polymer optical waveguide, a poly(methyl methacrylate) (PMMA) has ordinarily been used because of its high optical transparency. In addition to it, polyimides have particularly been expected because they have high glass transition temperatures (Tgs) and excellent thermal resistance and they can endure heat at soldering. However, in the case of an ordinary polyimide, since the C—H bonds of a benzene ring absorb near infrared light in a near infrared region (at a wavelength of from 1,000 to 1,700 nm) used for optical communication, the deuteration and fluorination of hydrogen atoms are necessary for securing optical transparency. For example, the Japanese Patent Application Laid-Open (JP-A) Nos. 3-72528, 4-328504, 2004-85937, and 2005-37841, and the Japanese Patent (JP-B) No. 2816771 disclose various fluorinated polyimides useful for the material of a polymer optical waveguide.
By the way, as described above, it is required to provide a relative refractive index difference between the core layer and the clad layer in the optical waveguide. Accordingly, for example, in the Japanese Patent Application Laid-Open (JP-A) Nos. 3-72528, and 4-328504, and the Japanese Patent (JP-B) No. 2816771, polyimides different in molecular chain skeleton are selected as the materials of both layers, or only the core layer is formed with a fluorinated polyimide and a material (e.g., a silicon oxide film or an air layer) other than polymers is used for the clad layer. In contrast, in the Japanese Patent Application Laid-Open (JP-A) Nos. 2004-85937 and 2005-37841, the core layer and the clad layer are formed with a fluorinated polyimide having basically the same molecular chain skeleton and a relative refractive index difference is provided by changing the fluorine contents of both layers.
Further, for example, the Japanese Patent Application Laid-Open (JP-A) No. 4-8734 discloses a method of controlling a relative refractive index difference by copolymerizing two kinds of fluorinated polyimides and adjusting the fluorine content of the resulting copolymer, and the Japanese Patent Application Laid-Open (JP-A) No. 6-51146 discloses a method of controlling a relative refractive index difference by irradiating electron beam to a fluorinated polyimide and adjusting its exposure dose.
However, these methods change the molecular chain skeleton of a fluorinated polyimide, control the fluorine content of a fluorinated polyimide, carries out the copolymerization of fluorinated polyimides, and carries out the irradiation of electron beams to a fluorinated polyimide, and therefore, they have problems that: they can be applied to only a fluorinated polyimide having a specific molecular structure; a large scale apparatus and complicated producing steps are required; and the like.
Thus, the Japanese Patent (JP-B) No. 3486325 discloses a method of obtaining partially fluorinated polyimide having a lowered refractive index by adding silica ultrafine particles to a partially fluorinated polyamide acid as a precursor, followed by heat treatment, as a method of simply controlling a relative refractive index difference using an ordinary fluorinated polyimide. Since the refractive index of quartz glass is about 1.45, it lowers a refractive index without lowering an optical transparency by dispersing silica ultrafine particles in a partially fluorinated polyimide.
However, the method of the Japanese Patent (JP-B) No. 3486325 has a problem that productivity is extremely bad because it is necessary to stir a mixture for 24 hours in order to improve dispersibility after adding a dispersed solution of silica ultrafine particles to a solution of a partially fluorinated polyamide acid.
By the way, the Japanese Patent Application Laid-Open (JP-A) No. 8-73739 discloses a production method of a polyimide composition in which silica particles are ultrafinely dispersed, characterized in that in the production method of a polyimide composition by which an alkoxysilane and/or its partially hydrolyzed condensate is reacted with a polyamide acid in the presence of water and polyimidation is subsequently carried out, the reaction is carried out in the presence of an acid catalyst and/or an amino group containing alkoxysilane. In this method, silanol is generated by the reaction of an alkoxysilane and/or its partially hydrolyzed condensate with water and a transparent polyimide composition (e.g., a polyimide film or sheet) having an excellent elastic modulus is provided by ultrafinely dispersing silica particles prepared from the silanol in the following heating step.
However, the Japanese Patent Application Laid-Open (JP-A) No. 8-73739 fails to disclose or suggest using a halogenated polyamide acid in place of a polyamide acid and is completely silent on drastically chaining a refractive index by ultrafinely dispersing silica particles while securing the appearance and transparency of the obtained halogenated polyimide film. Further, the polyimide composition disclosed in the Japanese Patent Application Laid-Open (JP-A) No. 8-73739 is described as useful for applications such as electronic materials, forming materials such as films and sheets, and coating materials, but not described as useful for optical materials.